U.S. officials said the working assumption was that the pair had been kidnapped for ransom.

Nigerian military officials, who deployed army and navy units in the hunt to find the kidnappers, as of late Thursday had no "hard information" on the whereabouts of them or the two American sailors taken hostage, a Nigerian Navy spokesman told NBC News.

The spokesman attributed the abductions to "criminals in the delta," emphasizing they were common criminals and pirates, not militants. Creeks and swamps leading to the Nigerian coast were being searched for the hostages.

The seized vessel is owned by Louisiana-based Edison Chouest Offshore, according to Reuters. The company was not immediately available for comment.

Sources told NBC News that there were no U.S. warships in the region and no immediate plans for a hostage rescue attempt. However, there is a contingent of U.S. Marines aboard a Dutch warship in the area as part of a military exchange program.

“We're obviously closely monitoring reports that two U.S. citizens have been kidnapped from a U.S. flagged vessel,” State Department spokeswoman Marie Harf said at a press briefing on Thursday. “It's a motor vessel, the C-Retriever, in the Gulf of Guinea. We are seeking additional information about the incident, so that we may contribute to safely resolving the situation.”

Satellite image of the coast of Nigeria.

“Obviously our concern at this point is for the safe return of the two U.S. citizens,” Harf said. “We do believe that this was an act of piracy. Again, we are continuing to seek additional information and for privacy reasons can't provide any additional information about the two U.S. citizens.”

In April 2009, U.S. Navy SEAL snipers killed three Somali pirates as they rescued American cargo ship Capt. Richard Phillips, who had offered himself as a hostage to save his crew.

The high-seas hijacking has been turned into a film starring Tom Hanks. "Captain Phillips" earned more than $52 million during its first two weeks in cinemas.

In an interview with NBC’s Brian Williams on Thursday, Phillips said the waters off Nigeria were “worse than even Somalia.”

“Wherever the opportunity for these thugs or pirates are, they will take advantage of it, and Nigeria is teeming right now,” Phillips added.

Phillips said the pirates were following a familiar pattern.

“This is the m.o. for the Nigerians. And they usually take a captain or a chief engineer and they’ll bring ‘em ashore and hide ‘em, so no rescue attempt can be made,” he said. “You have to understand, it is dangerous out in the Gulf of Guinea, but it's also dangerous on the land in Nigeria with everything going on there.”

The U.S.-flagged oil supply vessel C-Retriever, seen here in 2008, was attacked off the Nigerian coast on Wednesday.Courtesy Christian Serrano

Rory Lamrock, an analyst specializing in maritime security with U.K.-based risk-management firm AKE, said there had been "an increase in the severity of attacks in the Gulf of Guinea" over the past two years.

In August, Nigeria's navy killed 12 pirates as they tried to flee from a fuel tanker they had hijacked.

Earlier this month, the International Maritime Bureau reported that pirate attacks off Nigeria's coast had jumped by a third this year -- with 29 attacks on vessels recorded in the first nine months of 2013, up from 21 in the same period last year.

"Pirates, often heavily armed and violent, are targeting vessels and their crews along the [Nigerian] coast, rivers, anchorages, ports and surrounding waters," the IMB said. "In many cases, they ransack the vessels and steal the cargo."

The IMB said in the first nine months of 2013 the Gulf of Guinea accounted for all crew kidnappings worldwide, 32 of them off Nigeria, and two off Togo. In such incidents, sailors are taken ashore and usually held for ransom.

“Attacks by pirates off the coast of Nigeria in the Gulf of Guinea have increased substantially in recent years,” according to a June 2013 travel warning from the State Department. “Armed gangs have boarded both commercial and private vessels to rob travelers. The Nigerian Navy has limited capacity to respond to criminal acts at sea.”

Alistair Galloway, owner of private security provider Endeavour Maritime, said the discovery of new oil reserves in West African countries including Cameroon and Liberia was driving up shipping traffic in the area.

"This oil boom is attracting a far richer shipping environment and therefore more higher-value targets and that is the biggest threat to the region's maritime security," he said.

Galloway cited differences between West African pirates and their better-known Somalia-based counterparts.

"In Somalia, piracy was really out-of-work fisherman looking to improve their lives, but piracy is West Africa is really part of a bigger criminal system, networks embedded in the nations," he said. "The networks are in place and it’s been easy for them to attack."

Johan Potgieter, senior researcher at South Africa's Institute for Security Studies (ISS), said that pirates tended to feel "disenfranchised."

He added: "They feel that if the government won’t share its wealth equally, they will take it for themselves by other means."